Prof Sonia Antoranz Contera

Abstract:

In this work, we show atomic STM images of the layered compound Bi2Se3. We study the effect in the surface of the substitution of 5% and 20% of the Bi atoms for Sb in Bi1.9Sb0.1Se3 and Bi1.6Sb0.4Se3. The images of the three samples show similar trigonal structures corresponding probably to the van der Waals Se atoms. The distance measured between surface atoms in Bi2Se3 is 4.04 A, in Bi1.9Sb0.1Se3 is 4.16 A and in Bi1.6Sb0.4Se3 is 4.26 A. In Bi1.6Sb0.4Se3 some atomic sites appear brighter than others. The effect is accentuated at higher tunnelling currents and is not observed in the other compounds. Nanoscopic range depressions on the sample might be related to the skeletal crystal structure since the images show atomic corrugations that align slightly in one direction. We explain the results as the effects of the interactions between tip and sample, and discuss two interpretations: on the one hand, localised depression of the individual atomic sites, and on the other the possible elevation of the atoms of the surface due to a phase transition of the compounds induced by STM.

Abstract:

We have been able to raise squares of around 2 A in height of the layered materials Bi1.6Sb0.4Se3, Bi1.9Sb0.1Se3 and Bi2Se3 by means of contact atomic force microscopy in air. By raising squares on/beneath previously produced squares, Mayan-like pyramids have been constructed, each step of the pyramid being around 2 A in height. We neither remove matter from the surface nor produce a localized oxidation of the sample; we elevate a portion of it. When the surface is oxidized, the AFM cantilever pulls the oxide off producing holes. The raising of the squares is also possible in a nitrogen atmosphere.

Abstract:

Atomic force microscopy (AFM) has been used for tribological studies of silicon surfaces both with and without an oxide layer on the surface. Three different types of surfaces were prepared: a silicon surface with a chemical oxide made by the SC1 process, a silicon surface with a thermal oxide, and a H-terminated silicon surface without an oxide layer. Only in the case of the chemical oxide, scratching of the oxide and ploughing of the silicon by the Si3N4 AFM tip were observed. On the other hand, no wear of the sample was noted on the other surfaces. On these surfaces, the AFM often produced elevated patterns in the shape of the scanned area, which were no longer visible after HF etching. The difference between the tribological behavior of the chemical-oxide-covered surface and that of the other surfaces is discussed in relation to the presence of hydroxyl groups in the oxide layer.

Abstract:

The surface of the misfit-layer compounds (LaSe)xNbSe2 and (PbSe)xNbSe2 is studied by means of scanning tunneling microscopy (STM) and atomic force microscopy (AFM), in air and nitrogen, in the range of 0.5 to 10 μm2. The cleaved surface of (LaSe)xNbSe2 presents flat non-stepped surfaces which crack and react. Inner layers do not appear to crack. The dynamic process taking place in the superficial layers produces pieces which arrange in a self-similar way and eventually gives rise to discrete islands which self-organize periodically in one direction. On the other hand, the cleaved surface of (PbSe)xNbSe2 is stepped and shows no reactivity. The influence of the cleavage, the strain, the strength of the interlayer and intralayer bonding, and the misfit of the compounds are discussed as reasons for the mesoscopic behaviour of these materials.